How to calculate the city siege contribution?

[Chinesehero]

every time I do city siege my performance is normal and have dmg and heal
but sometimes get top 5 contribution sometimes the last 5
How is that???
Like just now I play 3 stages, 1 is 14 , 2 is 2, 3 is 18 <--this not add bonus yet
how is they calculate ?? why sometimes so good sometimes so bad???
and i see some dps got top dmg but they still cant go top 5